Molecular characteristics governing chlorine deposition and removal on promoted Ag catalysts during ethylene epoxidation

The selectivity of Ag ethylene epoxidation catalysts correlates with surface coverage of the promoter chlorine. Cl coverage, in turn, is a function of co-fed organic chloride and alkane moderator pressures. Organic molecules can be classified based on their deprotonation enthalpy, as those that are inert, those capable of selective oxidation, or those that act as gas-phase acids. The influence of alkane moderator and alkyl chloride promoter identities on Cl coverage was assessed by titration of Cl by ethane in a recirculating gas-phase batch reactor immediately following steady-state reactions with various alkane and alkyl chlorides included in ethylene oxidation feedstreams. The faculty of organic molecules for Cl removal correlates with the weakest C[sbnd]H bond dissociation enthalpy, such that molecules with C[sbnd]H bonds susceptible to homolysis remove Cl more effectively than those with stronger C[sbnd]H bonds. Cl deposition likely occurs via heterolytic cleavage of C[sbnd]Cl bonds in alkyl chlorides, such that molecules with weak C[sbnd]Cl bonds deposit multiple monolayers of Cl. Cl coverage affects ethylene epoxidation rates identically regardless of the organic chloride or the aliphatic hydrocarbon employed, demonstrating that these molecules only impact catalysis by moderating Cl coverage. These findings implicate the concurrent prevalence of homolytic and heterolytic pathways during ethylene epoxidation on promoted Ag/α-Al₂O₃ catalysts and provide context to examine additional roles of surface oxygen beyond involvement in oxidation pathways.